The antenna apparatus for use in a communication apparatus such as a remote keyless entry system for vehicular/home use that uses relatively short wavelength of UHF, VHF band has a large portion of its body volume occupied by an antenna element. Therefore, volume reduction of the antenna element is important in terms of volume reduction of the antenna apparatus.
Japanese patent document JP-A-2003-152427 discloses a volume reduction structure of the antenna apparatus. The disclosed structure has a linear inside conductor and an outer coil conductor that is densely wound at a distance from the inside conductor for providing specific resonance frequency. In this manner, the antenna device is aimed at achieving high gain with reduced body volume.
The structure disclosed in the above-identified document has the liner inside conductor, and that sets a limit for volume reduction. For example, to reduce the antenna size in a direction that is perpendicular to the extending direction of the inside conductor, at least one of the inside conductor and the outside coil conductor has to have an extended length. In this case, the linearity of the inside conductor contributes to the increase of the body volume by large amount.
On the other hand, Japanese patent document JP-A-2007-43653 (US2006/0290590) filed by the inventor of the present invention discloses a structure that has an inside conductor element in a spiral shape extending along an axis of the outside conductor element at an inside of the outside element, in which one of the elements serves as a signal line and the other serves as a GND line. In this manner, the inside element in a spiral shape achieves a narrow band for an improved gain, thereby enabling the body volume reduction for the same gain. Further, Japanese patent document JP-A-2007-221374 (US2007/0200786) discloses an antenna holder structure for holding the antenna apparatus on a substrate. In view of the above disclosure, further volume reduction is sought about for implementation efficiency.